A solid cable, which has been used as a submarine cable for transmitting electric power, is produced through the following process, for example. First, the outer circumference of the conductor is covered with an inner semiconducting layer. Insulating tapes (insulating-paper tapes) are lapped over the outer circumference of the inner semiconducting layer to form an insulating layer. An outer semiconducting layer is formed at the outside of the insulating layer to form a cable core. The cable core is taken up in a tank to undergo vacuum drying for removing the water in the insulating layer. Next, highly viscous insulating oil is introduced into the tank to pressure-impregnate the insulating layer with the oil. Subsequently, the insulating layer is covered with a metal sheath, which is then covered with an anticorrosion covering. Furthermore, a steel-wire armoring, a protecting layer, and so on are formed at the outside of the anticorrosion covering. An example of the structure of the foregoing solid cable is shown in FIG. 5 (the steel-wire armoring and protecting layer are omitted). This cable has, from the center in this order, a conductor 1, an inner semiconducting layer 2, an oil-impregnated insulating layer (hereinafter referred to as an insulating layer) 3, an outer semiconducting layer 4, a metal sheath (made of lead or the like) 5, and an anticorrosion covering (made of polyethylene or the like) 6.
A solid cable, which has the above-described structure, has been used in which the insulating layer 3 is formed by using kraft-paper tapes, which are free from swelling by the insulating oil, and is impregnated with relatively highly viscous insulating oil (high-viscosity oil). However, the viscosity of the insulating oil decreases as the temperature rises. As a result, the maximum operating temperature of the solid cable has been limited to 55° C. or so. In this type of cable, under a condition that a load is applied (the ON condition) and the conductor is at the maximum operating temperature, when the load is interrupted (the OFF condition), the high-viscosity oil, which has expanded and moved to the outside of the insulating layer, cannot follow the temperature decrease sufficiently. As a result, the pressure at the inner-side portion of the insulating layer (the portion in the vicinity of the conductor), in particular, becomes negative, forming voids at that portion. Usually, the cable is designed such that the void formation does not affect the insulating performance. Consequently, the operating temperature is limited to 55° C. or below. When the maximum operating temperature is increased to a temperature higher than 55° C. in order to increase the transmission capacity, electrical discharge at the voids formed at the time of the load interruption (the OFF condition) can cause insulation breakdown. As a result, it has been difficult to meet the requirement of increasing the operating temperature and transmission capacity in order to realize long-distance large-power transmission.
To solve the above-described problem, a solid cable and a production method thereof have been proposed (see Patent Literature 1, for example). This cable not only uses, as the insulating oil with which the insulating layer 3 is impregnated, medium-viscosity insulating oil (medium-viscosity oil) having a viscosity of 10 cst or more and less than 500 cst at 60° C. but also uses insulating tapes, each of which includes a polyolefin-family resin film, in at least one portion of the insulating layer 3. Because the foregoing solid cable uses medium-viscosity oil having a viscosity lower than that of the high-viscosity oil, the variation in the oil volume resulting from the temperature variation can be suppressed. Consequently, the insulating oil having moved to the outer-radius side of the insulating layer at the time the load is applied is likely to return to the inner-radius side at the time the load is interrupted. Thus, the formation of the voids that can lead to the insulation breakdown is suppressed. Therefore, it can be expected that the cable satisfies the requirement of increasing the operating temperature and transmission capacity by exploiting the above-described features.    Patent Literature 1: the published Japanese patent application Tokukaihei 11-224546